It is well known that phyllosilicates, such as smectite clays, e.g., sodium montmorillonite and calcium montmorillonite, can be treated with organic molecules, such as organic ammonium ions, to intercalate the organic molecules between adjacent, planar silicate layers, thereby substantially increasing the interlayer (interlaminar) spacing between the adjacent silicate layers. The thus-treated, intercalated phyllosilicates, having interlayer spacings of at least about 5 Angstroms, preferably at least about 10-20 Angstroms and up to about 100 Angstroms, then can be exfoliated, e.g., the silicate layers are separated, e.g., mechanically, by high shear mixing. The individual silicate layers, when admixed with a matrix polymer, before, after or during the polymerization of the matrix polymer, e.g., a polyamide--see U.S. Pat. Nos. 4,739,007; 4,810,734; and 5,385,776 --have been found to substantially improve one or more properties of the polymer, such as mechanical strength and/or high temperature characteristics.
Exemplary of such prior art composites, also called "nanocomposites", are disclosed in published PCT disclosure of Allied Signal, Inc. WO 93/04118 and U.S. Pat. No. 5,385,776, disclosing the admixture of individual platelet particles derived from intercalated layered silicate materials, with a polymer to form a polymer matrix having one or more properties of the matrix polymer improved by the addition of the exfoliated intercalate. As disclosed in WO 93/04118, the intercalate is formed (the interlayer spacing between adjacent silicate platelets is increased) by adsorption of a silane coupling agent or an onium cation, such as a quaternary ammonium compound, having a reactive group which is compatible with the matrix polymer. Such quaternary ammonium cations are well known to convert a highly hydrophilic clay, such as sodium or calcium montmorillonite, into an organophilic clay capable of sorbing organic molecules. A publication that discloses direct intercalation (without solvent) of polystyrene and poly(ethylene oxide) in organically modified silicates is Synthesis and Properties of Two-Dimensional Nanostructures by Direct Intercalation of Polymer Melts in Layered Silicates, Richard A. Vaia, et al., Chem. Mater., 5:1694-1696(1993). Also as disclosed in Adv. Materials, 7, No. 2: (1985), pp, 154-156, New Polymer Electrolyte Nanocomposites: Melt Intercalation of Poly(Ethylene Oxide) in Mica-Type Silicates, Richard A. Vaia, et al., poly(ethylene oxide) can be intercalated directly into Na-montmorillonite and Li-montmorillonite by heating to 80.degree. C. for 2-6 hours to achieve a d-spacing of 17.7 .ANG.. The intercalation is accompanied by displacing water molecules, disposed between the clay platelets with polymer molecules. Apparently, however, the intercalated material could not be exfoliated and was tested in pellet form. It was quite surprising to one of the authors of these articles that exfoliated material could be manufactured in accordance with the present invention.
Previous attempts have been made to intercalate water-soluble polymers, such as polyvinylpyrrolidone (PVP), polyvinyl alcohol (PVA) and poly(ethylene oxide) (PEO) between montmorillonite clay platelets with little success. As described in Levy, et al., Interlayer Adsorption of Polyvinylpyrrolidone on Montmorillonite, Journal of Colloid and Interface Science, Vol. 50, No. 3, March 1975, pages 442-450, attempts were made to sorb PVP (40,000 average M.W.) between monoionic montmorillonite clay platelets (Na, K, Ca and Mg) by successive washes with absolute ethanol, and then attempting to sorb the PVP by contact with 1% PVP/ethanol/water solutions, with varying amounts of water, via replacing the ethanol solvent molecules that were sorbed in washing (to expand the platelets to about 17.7 .ANG.). Only the sodium montmorillonite had expanded beyond a 20 .ANG. basal spacing (e.g., 26 .ANG. and 32 .ANG.), at 5.sup.+ % H.sub.2 O, after contact with the PVP/ethanol/H.sub.2 O solution. It was concluded that the ethanol was needed to initially increase the basal spacing for later sorption of PVP, and that water did not directly affect the sorption of PVP between the clay platelets (Table II, page 445), except for sodium montmorillonite. The sorption was time consuming and difficult and met with little success.
Further, as described in Greenland, Adsorption of Polyvinyl Alcohols by Montmorillonite, Journal of Colloid Sciences, Vol. 18, pages 647-664 (1963), polyvinyl alcohols containing 12% residual acetyl groups could increase the basal spacing by only about 10 .ANG. due to the sorbed polyvinyl alcohol (PVOH). As the concentration of polymer in the intercalant polymer-containing solution was increased from 0.25% to 4%, the amount of polymer sorbed was substantially reduced, indicating that sorption might only be effective at polymer concentrations in the intercalant polymer-containing composition on the order of 1% by weight polymer, or less. Such a dilute process for intercalation of polymer into layered materials would be exceptionally costly in drying the intercalated layered materials for separation of intercalate from the polymer carrier, e.g., water, and, therefore, apparently no further work was accomplished toward commercialization.
In accordance with an important feature of the present invention, it has been found that it is best to exfoliate the phyllosilicate prior to contacting the phyllosilicate with a polymer or oligomer, to provide polymer or oligomer-bonded platelets that do not include a substantial quantity of non-exfoliated phyllosilicate clumps or tactoids. Best results are achieved using a water-insoluble oligomer (herein defined as a pre-polymer having 2 to about 15 recurring monomeric units, which can be the same or different) or water-insoluble polymer (herein defined as having more than about 15 recurring monomeric units, which can be the same or different) composition for intercalation having at least about 2%, preferably at least about 5% by weight oligomer or polymer concentration, more preferably about 50% to about 80% by weight oligomer and/or polymer, based on the weight of oligomer and/or polymer and carrier (water and a solvent for the oligomer or polymer) to achieve better sorption of the polymers or oligomers onto the external surfaces of the exfoliated phyllosilicate platelets. The water-insoluble oligomer or polymer is sorbed onto at least one outer surface of the silicate platelets.
A phyllosilicate, such as a smectite clay, can be easily and completely exfoliated by adding water and an organic solvent having a polar functionality, and then shearing the phyllosilicate/water/solvent mixture. After exfoliation, it has been found that a water-insoluble polymer and/or a water-insoluble oligomer can be adhered to the outer surfaces of the exfoliated platelets, so long as the polymer or oligomer has a carbonyl, hydroxyl, carboxyl, amine, amide, ether, ester, sulfate, sulfonate, sulfinate, sulfamate, phosphate, phosphonate, or phosphinate functionality, or an aromatic ring to provide metal cation bonding, via a metal cation of the phyllosilicate sharing electrons with two electronegative atoms of two functional groups of polymer molecules, to the outer surfaces of the exfoliated phyllosilicate platelets. The electronegative atoms can be, for example, oxygen, sulfur, nitrogen, and combinations thereof. Atoms having a sufficient electronegativity to bond to metal cations on the inner surface of the platelets have an electronegativity of at least 2.0, and preferably at least 2.5, on the Pauling Scale. A "polar moiety" or "polar group" is defined as a moiety having two adjacent atoms that are bonded covalently and have a difference in electronegativity of at least 0.5 electronegativity units on the Pauling Scale. Such polymers have sufficient affinity for the phyllosilicate platelets to maintain sufficient interlayer spacing for exfoliation, without the need for coupling agents or spacing agents, such as the onium ion or silane coupling agents disclosed in the above-mentioned prior art. A schematic representation of the charge distribution on the surfaces of a sodium montmorillonite clay is shown in FIGS. 1-3. As shown in FIGS. 2 and 3, the location of surface Na.sup.+ cations with respect to the location of oxygen (Ox) Mg, Si and Al atoms (FIGS. 1 and 2) results in a clay surface charge distribution as schematically shown in FIG. 3. The positive-negative charge distribution over the entire clay surface provides for excellent dipole/dipole attraction of the polar moieties of the polymer molecules on the surfaces of the clay platelets.
Sorption and metal cation electrostatic attraction or bonding of a platelet metal cation between two oxygen or nitrogen atoms of the polymer or oligomer molecules; or the electrostatic bonding between the interlayer cations in hexagonal or pseudohexagonal rings of the smectite platelet layers and an oligomer or polymer aromatic ring structure provides tenacious bonding between the polymer or oligomer and the outer surface of the exfoliated silicate platelets or other layered material, while providing individual silicate/polymer nanocomposite material, and without including a substantial quantity of non-exfoliated clumps or tactoids of phyllosilicate material. Phyllosilicates easily can be exfoliated into individual phyllosilicate platelets before admixture with a polymer or oligomer, as well known in the art.